Tow mechanisms underline the regulation of the immune response and will be studied: 1. Role of serum antibody. 2. Role of cell surface antibody. The energetics of interaction of antigen and specifically reactive immunocytes in the presence of serum antibody will be further studied. The model system employed is the reaction between DNP conjugates (both multivalent and univalent) and MOPC 315 murine myeloma cells which have a portion of the immunoglobulin which is reactive with DNP on their cell surface. The capacity for these conjugates to interact with the cells even in the presence of a large excess of MOPC 315 myeloma protein will be further explored. These studies should lead to a better understanding of how a small amount of immunogen can stimulate an immune response in the presence of a large excess of specific antibody and will have a bearing on the mechanism by which immunogen initiates stimulation and replication of immunocytes. In addition, we will pursue the observation of an IgD-like molecule on the surface of murine lymphoid cells. This molecule appears to play a regulatory role in regards to the immune response. The ontology of the development of this surface molecule and the influence of antigenic exposure and the presence of a thymus will be studied to determine the function of this particular class of membrane antibody in mice.